Conventional heat exchangers employed in refrigerators and air-conditioning apparatuses include those which are called fin and tube heat exchangers. One such heat exchanger is constituted by: plate-shaped fins that are arranged at a fixed interval and between which gas (air) passes through; and heat transfer tubes that are inserted at right angle through these plate-shaped fins (hereinafter, simply referred to as “fins”) and through which a refrigerant flows. Known factors of influence on the heat transfer performance of this fin and tube heat exchanger include a heat transfer coefficient on the refrigerant side between the refrigerant and the heat transfer tubes, a contact heat transfer coefficient between the heat transfer tubes and the fins, and an air-side heat transfer coefficient between the air and the fins.
In order to increase the heat transfer coefficient on the refrigerant side between the refrigerant and the heat transfer tubes, performance inside the tubes is facilitated by increasing area of the heat transfer tubes and by cutting inner grooves, which allows a stirring effect of the refrigerant to be obtained, in the heat transfer tubes. Furthermore, in order to enhance the air-side heat transfer coefficient between the air and the fins, slit groups, which are formed by performing cutting and raising of the fins, are provided between adjoining heat transfer tubes. These slit groups are provided so that the edges of the slits face the wind direction. By thinning the hydrodynamic boundary layer and the thermal boundary layer of the air flow at these edges, heat transfer is facilitated and heat exchange capacity is increased. Furthermore, the contact heat transfer coefficient between the heat transfer tubes and the fins are influenced by the contact condition between the heat transfer tubes and the fins.
For example, as illustrated in FIG. 8, when a heat transfer tube 10 is expanded and is fixed to fins 1, there occurs, between the outer surface of the heat transfer tube 10 and the fins 1, gaps caused by waviness of the outer surface of the heat transfer tube 10, gaps caused by deformation of the intermediate portion of a fin collar 2, and a gap between a fin 1 and a fin 1. The drop in contact heat transfer coefficient owing to these gaps is considered to be about five percent of the heat exchanger (see Non Patent Literature 1, for example).
Accordingly, in order to reduce these gaps and increase the contact heat transfer coefficient, a technique has been proposed, for example, as illustrated in FIG. 9, in which three or more bends R are provided for the fin collar 2 of the fin 1 along which the heat transfer tube 10 is inserted. In this technique, further, the bends R are smoothly connected to each other, the fin collar 2 is generally shaped to convex to the heat transfer tube 10 side, with no straight portion existing (See Patent Literature 1).